Question

when Sally picks any number between 1 and 20, doubles it, adds 6, divides by 2 and subtracts 3, she always gets the number she started with. why?

Answer 1

I think it might be because 2 and 3 are factors of 6. 

Answer 2

lets say number is x

doubles it (2x)
adds 6 (2x+6)
divide it by 2 (2x+6=2(x+3) so 2(x+3) divided by 2=x+3)
subtract 3 (x+3-3=x)

becasue math is fun, brah